1. Technical Field
This invention relates to multi-layer microstrip structures and, more particularly, to superconducting multi-layer microstrip structures with at least one dielectric layer made of benzocyclobutene deposited adjacent a superconducting layer.
2. Background
Microwave and radio frequency (RF) circuits typically require passive circuit components, such as filters, delay lines, couplers, etc., which are based on transmission lines. Multi-chip modules (MCMs) , which require high density, low attenuation interconnect wiring between chips, share many of the technical requirements of the passive circuit components for microwave circuits.
Waveguide transmission lines or waveguides, which have low attenuation and high frequency capabilities, were initially employed for microwave circuits. However, more compact transmission lines were desired. Use of microstrip and stripline structures instead of waveguides provided both size and weight savings in RF and microwave circuits. Typically such microstrip and stripline structures include metal wiring, such as copper, gold or silver, deposited on a substrate made from ceramic, epoxy or crystal. For example see K. C. Gupta, R. Garg, and I. Bahl, Microstrip Lines and Slotlines (Artech House Inc. 1979), hereby incorporated by reference. However, the metal microstrip wiring suffers from resistive losses which adversely affect insertion loss and prohibit use of the metal microstrip wiring in narrowband microwave filters. Compactness of the metal microstrip wiring is limited since the resistive losses increase as the width and height of the metal microstrip wiring decreases. Resistive losses and line attenuation also limit wiring density for MCMs.
To alleviate the resistive losses, superconducting films have been employed. For example, see R. S. Wither, "Passive Microwave Devices and Their Applications", The New Superconducting Electronics (Klewer Academic Publishers 1993), hereby incorporated by reference. Two superconducting transmission line approaches have heretofore been employed namely, substrate-based microstrip and deposited thin film. Substrate-based microstrip are typically 30-500 microns thick and rely on a substrate material to act as an insulating dielectric for the transmission line. Deposited thin film microstrips are typically 0.01 to 1 micron thick and require a groundplane to be deposited on top of the substrate. All conducting and insulating layers of the transmission line are built on top of the supporting substrate. While the deposited thin film approach produces more compact devices than either the waveguide or substrate-based microstrip, the thinness of the dielectric limits power handling capacity, reduces efficient coupling and restricts lithographically practical impedance ranges.
A superconducting multi-layer process with low resistive losses has been developed using mullite and polyimide. See S. Tanahashi et al. "Superconducting Multichip Module for Josephson Computer Circuits", Fourth International Superconductive Electronics Conference (1993), which is hereby incorporated by reference. The multi-layer process has its drawbacks due to the high processing temperatures required for curing the resins. This reduces the current carrying capacity of the superconducting films by a factor of 5.
What is needed is a compact microstrip structure with low attenuation, low resistance losses, efficient coupling characteristics, controllable impedance and high power handling capability.